Atomicity
Reading: 9.1-9.2.2, 9.3-9.4.1, 9.5-9.6,
Overview¶
An action that changes several data values can have any or all of at least four independent properties:
- all-or-nothing (either all or none of the changes happen)
- before-or-after (the changes all happen either before or after every concurrent action)
- constraint-maintaining (the changes maintain some specified invariant)
- durable (the changes last as long as they are needed).
failure masking, coordination
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scenario 1: the processor instruction set interface as seen by a thread in the interrupt interface
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signal arriving from outside the thread’s interpreter
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interrupted thread, interrupt thread, interrupt handler
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Ex, an I/O operation that the other thread was waiting for may now have completed.
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sth wrong in the interrupted thread
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exception handler, in the orginal thread
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Ex, deadlock, hw/sw faults, user side, the running thread encountered a missing-page exception in a virtual memory system
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scenario 2: a register overflow at the machine level in a layered application
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Write intelligible error message in each layer
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A higher layer will hide the lower layers
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All-or-nothing action means an action either completes or backs out.
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missing-page exception occurs halfway of an instruction (pipelined processor)
- How to save the next-instruction pointer?
- every instruction is atomic -> the next-instruction pointer = addr of instruction that encountered the missing page
- if one instruction is not atomic, the exception will still happen in that instruction
- supervisor call (SVC) to a kernel READ procedure, keystroke to the caller
- blocking read
- non-blocking read
- without all-or-nothing read
- How to save the next-instruction pointer?